The cell is the irreducible element of life and is often studied as a living model of complex biological systems. Cells are often studied in vitro, i.e. in culture, in a homogeneous medium either suspended (anchorage independent cells) or attached to a surface (anchorage dependent cells). The majority of mammalian cells are anchorage dependent, i.e. adherent. In culture, they grow in layers usually attached to plasticware (tissue culture polystyrene) in cell culture growth media under controlled conditions (incubators with humidified atmosphere at 37° C. with 5% CO2). Adherent cells often divide until the surface on which they are attached is fully covered—they divide and cumulate to form a confluent monolayer. Upon reaching confluency, cells are subcultured (i.e., passaged, split), by harvesting and splitting the population of cells into smaller populations that are further cultured. Some cultures are semi-adherent, and grow as a mixed population where a proportion of cells does not attach to the tissue culture flask and remains in suspension. To maintain this heterogeneous population both the attached cells and the cells in suspension must be subcultured.
Cell-based assays are conventionally performed in well plates that enable simultaneous analysis of multiple cell types or stimuli. For such multiplexed analyses, cells are cultured and assayed in wells holding microliter-milliliter volumes. Cell response to stimuli in well plates is often evaluated using microplate readers, which can be integrated with fluid handling and other miscellaneous equipment in a robotic analysis platform. A major drawback of such systems is the expense of the instrumentation and the experimental consumables (e.g., plates, pipette tips, reagents, and cells). The latter is a particular disadvantage for cell-based assays as they are generally more complex and require larger amounts of reagents than cell-free assays.1 
Recently, microfluidics has been touted as a solution to the challenges inherent in conducting multiplexed cell-based assays.2 The conventional format for microfluidics, which is characterized by devices containing networks of micron-dimension channels, allows integration of multiple processes on a single platform while reducing reagent consumption and analysis time. There are numerous advantages of using microfluidic based systems for cell assays, some of which are self-similarity in dimensions of cells and microchannels (10-100 μm widths and depths), laminar flow dominance and formation of highly resolved chemical gradients, subcellular delivery of stimuli, reduced dilution of analytes, and favorable scaling of electrical and magnetic fields. For the last ten years, researchers have used microchannels to manipulate and sort cells, to analyze cell lysates, to assay intact-cell biochemistry, and to evaluate cell mechanical and electrical responses. In most of these studies, cells were exposed to one stimulus or to a limited number of stimuli.
There have been just a few attempts to conduct multiplexed assays as it is difficult to control many reagents simultaneously in a complex network of connected channels, even when using microvalve architectures developed for microfluidic devices.3 In addition, there have been only a few microfluidic devices integrated to multiplexed detection instruments such as microplate readers;4 we believe this will be a necessary step for the technology to become competitive with robotic screening systems. Finally, we note that advancements have been made towards long-term cell culture and cell assays in microfluidic systems; however, there are no reports on passaging cells in microchannels or in other microsystems.
A potential solution to the limitations of the channel-microfluidic format is the use of “digital” or droplet-based microfluidics. In digital microfluidics (DMF), discrete droplets containing reagents are manipulated by sequentially applying potentials to adjacent electrodes in an array.5-14 Droplets can be manipulated independently or in parallel on a reconfigurable path defined by the electrode actuation sequence, which allows for precise spatial and temporal control over reagents. As with all microscale techniques, cross-contamination is a concern for DMF, but this phenomenon can be avoided by dedicating separate paths for each reagent. DMF has been used to actuate a wide range of volumes (nL to μL) and, unlike channel devices, there is no sample wasted in creating small plugs for analysis. In addition, each droplet is isolated from its surroundings rather than being embedded in a stream of fluid—a simple method of forming a microreactor in which there is no possibility that products will diffuse away. The preservation of products in a droplet is of great importance in cell assays targeting molecules secreted from cells into extracellular space. In addition, droplets provide mostly static fluid conditions without unwanted shear stress that is inevitable in continuous flow microfluidics. A further advantage of DMF is its capacity to generate nanoliter samples by translating droplets through selective wettability areas on an electrowetting-based platform.15 
There is currently much enthusiasm for using DMF to implement multiplexed assays; however, it has only been applied to a few non-cell assays. To the inventors' knowledge, there are no reports of the use of DMF to analyze cells. There are a few studies demonstrating only dispensing and manipulation of droplets containing cells, cell sorting, and cell concentration on a DMF platform. WO 2007/120241 A2 entitled “Droplet-Based Biochemistry”16 discloses dispensing and dividing droplets containing cells, generating droplets with single cells, detecting a type of cell, and sorting cells. US20070148763 A1 entitled “Quantitative cell dispensing apparatus using liquid drop manipulation”17 describes cell droplet handling, to achieve a predetermined number of cells. In a journal paper by Fan et al,18 dielectrophoresis was used to concentrate neuroblastoma cells within droplets on a DMF platform.
It would be very advantageous to provide droplet-based cell culture and/or assays using digital microfluidics in order to enable automated cell micro-culture and high-throughput screening ability for cell analysis. DMF has a high potential to address many problems associated with standard culture and assaying in well-plates or in continuous-flow microfluidic devices.